Prosthetic human body implants can be anchored in bone through a variety of different techniques such as friction fit, threads, porous areas for tissue ingrowth and bone cements. The use of a bone cement such as polymethyl methacrylate (PMMA) is the most popular technique for implant fixation even though when cured most cements have a substantially lower strength than that of cortical bone. When the strength of cement are exceeded, the cement tends to crack and cause the prosthetic device to become loosened.
These strength limitations are especially troublesome for the femoral stem portion of a hip prosthesis where tensile strains of 1000 microstrain can routinely occur. Cyclic strains of this magnitude can tend to break down a typical PMMA fixated stem due to fatigue cracking during as little as 1,000,000 repetitions, which can typically occur in a year's time. Moreover, a single loading of event that creates a tensile load in excess of about 9,000 pounds per square inch has been found to fracture PMMA. Stress studies indicate that the fatigue limits of PMMA bone cement can be exceeded in normal daily activities, such as walking or climbing stairs. Moreover, sudden impacts such as a fall can greatly exceed the tensile limits of PMMA bone cement and shatter or fracture the cement instantaneously.
PMMA bone cement has a compressive strength that is 50-70% of that of cortical bone; one-tenth the modulus of elasticity; one-fourth the tensile strength; 60% of the shear strength; and less than one-half the fatigue strength. These limitations have for years restricted the use of cemented prosthetic devices to patients that would not be particularly active after receiving such an implant.
Attempts have been made to increase the static and fatigue integrity of bone cement that include the addition of randomly oriented fibers formed of materials such as Kevlar, carbon, metal and other fibers. The use of such fibers has not proven satisfactory because intrusion properties of the bone cement are adversely affected and the fibers tend to shed and migrate throughout the body.
Other techniques such as ultrasonic and vacuum mixing, changing the powder to liquid ratio and centrifugation have also been used. The latter technique has generally been regarded as providing the most important improvement in PMMA bone cement over the past 25 years since it tends to reduce cement porosity.
A precoat of bone cement to the prostheses has also been proposed and although it does improve substrate-cement interface strength between the prosthesis and cement, the precoat does not improve static, fatigue and impact resistance.
As a result of these difficulties in developing a prosthesis for active patients that can be anchored through the use of bone cement, other techniques are actively being pursued at the present time. These include various surface configurations for providing a better mechanical fit between the prosthesis and cavity and porous coatings that promote tissue ingrowth. However, these proposed solutions are presently undergoing development and have not resulted in an implant that is universally acceptable for all applications. Although the use of ingrowth promoting surfaces is generally regarded as having great promise, these types of devices tend to be significantly more expensive than those that are designed to be anchored through the use of bone cement and require a longer recovery time before the patient can become active.